You're probably right, it would be too much to hope for. But I don't see it as 'oh, all these movies happened in the same universe'- since the bay films were a reboot, that is impossible. What the intent more is is that 'yeah, the turtles fought Chrome Dome, and stopped Shredder and his evil businessman ally from poisoning the city', or 'we stopped shredder, krang, rocksteady, and bebop from bringing the technodrome through to earth'. It's more an in-universe acknowledgement of the franchise's history, the good and the bad. And an indication that the new continuity of turtles we are following have ther own history of great battles. A history that viewers of the six films would be familiar with, even if it's not exactly the same.

That's probably way more doable...but again very ambitious. Maybe if they set it up right it could be done this way and it would set up for different TMNT movies but they way most lay people view the franchise it'd be shooting for a lot, just in my opinion.